The present invention relates to admission control in a mobile radio communications system.
In a mobile radio communications system, a mobile radio station communicates over an assigned radio channel with a radio base station. Several base stations are connected to a switching node which is typically connected to a gateway that interfaces the mobile communications system with other communication systems. A call placed from an external network to a mobile station is directed to the gateway, and from the gateway through one or more switching nodes to a base station which serves the called mobile station. The base station pages the called mobile station and establishes a radio communications channel. A call originated by the mobile station follows a similar path in the opposite direction.
In a spread spectrum, Code Division Multiple Access (CDMA) mobile communication system, spreading codes are used to distinguish information associated with different mobile stations or base stations transmitting over the same frequency band. In other words, individual radio xe2x80x9cchannelsxe2x80x9d correspond to and are discriminated on the basis of these codes. Various aspects of CDMA are set forth in one or more textbooks such as xe2x80x9cApplications of CDMA and Wireless/Personal Communications,xe2x80x9d Garg, Vigay K. et al., Prentice-Hall 1997.
Spread spectrum communications permit mobile station transmissions to be received at two or more xe2x80x9cdiversexe2x80x9d base stations and processed simultaneously to generate one received signal. With these combined signal processing capabilities, it is possible to perform a handover from one station to another without any perceptible disturbance in the voice or data communications. This kind of handover is typically called diversity handover and may include xe2x80x9csoftxe2x80x9d handover between two base stations and xe2x80x9csofterxe2x80x9d diversity handover between two different antenna sectors connected to the same, multi-sectored base station.
The signal propagation loss between a radio transmitter and receiver varies as a function of their respective locations, obstacles, weather, etc. As a result, large differences may arise in the strength of signals received at the base station from different mobiles. In addition, signals received by a base station from a mobile station close to the base station are much stronger than signals received from other mobile stations located in the base station""s cell boundary. Because all users of a CDMA communications system transmit information using the same frequency band at the same time, each user""s communication interferes with the communications of the other users. In fact, if one mobile station transmits at a power output that is too large, the interference it creates degrades the signal-to-interference ratio signals received from other mobile radios to the point that the receiving base station cannot correctly demodulate transmissions from the other mobile radios. Another problem with transmitting with too much power is the so-called xe2x80x9cparty effect.xe2x80x9d If the mobile transmits at too high of a power level, the other mobiles may increase their respective power levels so that they can xe2x80x9cbe heardxe2x80x9d compounding the already serious interference problem.
Thus, while transmit power control is important in any mobile radio communications system, it is particularly important to the performance and capacity of a CDMA-based mobile radio communications system. Two radio-related parameters, among others, affect the capacity of a CDMA-based system: the uplink (from mobile station-to-base station) interference received in a radio base station and the downlink (from base station-to-mobile station) power sent out by the radio base station. The uplink interference level includes the sum of all radio beams that reach a receiver in the base station for a specific radio frequency carrier. Such radio beams include those transmitted by mobile stations that are located in cells assigned to that base station or in neighboring cells handled by other base stations. Of course, additional noise and pseudo-noise contributes to the received interference level.
Because of the importance of these parameters to the capacity of the radio network, a radio network controller receives measurement reports from one or more radio base stations including periodic uplink interference and downlink power measurements. These measurement reports may be used by a call admission control function of the radio network controller. For example, in response to requests to set up a new call connection or to add radio resources to an existing call connection to a mobile station, the call admission control function determines, for example, if current interference levels in a base station cell where the mobile station is located permit the addition of the new call or the addition of new resources to an existing call. If the downlink power and uplink interference levels are sufficiently low, the admission control function may xe2x80x9cadmitxe2x80x9d the new call request and allocate the appropriate radio resources assuming other conditions are met, e.g., there are sufficient radio resources currently available.
In practice there is a delay between the time when a request for additional radio resources or a request to release radio resources is granted and the time when the effect of that radio resource grant/release is reflected in the base station measurement reports provided to the radio network controller. As a result of this delay, there is a risk that admission control decisions are based on old measurement values that are no longer accurate. This risk is heightened when many requests for (or releases of) radio resources come at substantially the same time, e.g., when a subway train with many users enters a new cell.
It is an object of the present invention to overcome the above-described difficulties and to provide a more accurate admission control procedure based on measurement reports made by the radio base stations.
It is another object of the present invention to provide an admission control algorithm that compensates for delays between radio resource grants/releases and the time when the effect of such grants/releases are reflected in base station measurement reports using predicted measurement values.
It is a further object of the present invention to provide an admission control memory that permits combination of actual measurement values with predicted measurement values resulting in more accurate admission control.
These and other objects are accomplished using admission control procedures in accordance with the present invention. Initially, a measurement value of a radio-related parameter is obtained. The effect that a requested radio resource or a recently allocated or de-allocated radio resource will likely have on the measurement value of the radio-related parameter is estimated or predicted. A determination is made whether to allocate additional radio resources based on the measurement value and on the estimated effect. The estimation time period is based on an interval between the time of allocating/de-allocating a radio resource and the time of detecting the effect of that allocated or de-allocated resource on the measurement value. Alternatively, the estimation time period may be based on a previously-allotted measurement time cycle.
In a non-limiting, example embodiment, one radio-related parameter is uplink interference at the radio base station. Another parameter may be downlink base station transmit power. Alternatively, measurement values of plural radio-related parameters, e.g., both uplink interference and downlink base station transmit power, may be employed. The measurement value(s) is (are) stored along with a corresponding estimated or predicted value. Corresponding stored values are combined to provide a modified measurement value from which a more accurate resource allocation decision can be made. The estimated or predicted value is removed, replaced, or ignored in the admission control memory after the estimation time period expires.
As a result of the admission control procedures in accordance with the present invention, the quality of already-established call connections is not adversely impacted by the allowance of new calls or the allocation of additional resources in already overloaded situations. This kind of predicted control is especially significant where many new call setup or other requests/releases for radio resources are received within a very short time period. Admission control is also improved in close to overload situations, and again, particularly when many call resource allocation/de-allocation requests are received in a relatively short time period. Consequently, the predictive nature of the present invention permits the admission control to more accurately maximize the number of calls that can be setup without undesirable performance degradation.